Magnetic Force Microscopy (MFM)

Magnetic Force Microscopy (MFM) makes use of Interleave Scanning and LiftMode functionality. Ensure that you are familiar with these techniques prior to attempting MFM applications.

LiftMode allows imaging of relatively weak but long-range magnetic interactions while minimizing the influence of topography. In MFM, a tapping cantilever equipped with a modified tip first scans over the sample surface to obtain topographic information. Using LiftMode, the tip is then raised above the sample surface. The surface topography from the initial scan is added to the lift height to maintain constant separation during the lifted scan. Magnetic interactions are detected during this second pass. In LiftMode, topographical features are virtually absent from the MFM image. This process is illustrated below:

The influence of magnetic force is measured using the principle of force gradient detection.

Force Gradient Detection

In the absence of magnetic forces, the cantilever has a resonant frequency f₀. This frequency is shifted by an amount Δf proportional to vertical gradients in the magnetic forces on the tip. The shifts in resonant frequency tend to be very small, typically in the range 1–50 Hz for cantilevers with a resonant frequency f₀ ~100 kHz.

These frequency shifts can be detected three ways:

• Phase Detection, which measures the cantilever’s phase of oscillation relative to the piezo drive
• Amplitude Detection, which tracks variations in oscillation amplitude
• Frequency Modulation, which directly tracks shifts in resonant frequency

Phase detection and frequency modulation produce results that are generally superior to amplitude detection. Amplitude detection has largely been superseded by frequency modulation and phase detection.

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